Single frequency (SF) in-band signalling systems are widely utilized in telephone equipment to pass address, supervisory, and alerting signals for telephone trunks or lines over 4-wire voice frequency transmission facilities without impairing their normal use for voice signals. An SF tone detection circuit is required at each end of the system to detect the normal standard signalling tone of 2600 Hz in the presence of voice and other signals being transmitted on the transmission facility.
Prior tone detection circuits utilized a combination of pass-band and stop-band filters with a moderately high Q to separate the tone signal frequencies from those of the guard signal frequencies (i.e. those which are present in the balance of the voice frequency spectrum). A general requirement is that the magnitude of the tone signals be 10 dB greater than that of the guard signals. However this 10 dB differential becomes severely distorted as the guard signals approach 2600 Hz, due to the rolloff characteristics of the filters. For example, when the guard signal is 100 Hz away from 2600 Hz, the guard signal is attenuated approximately 10 dB. As a result the detector indicates the presence of a valid SF tone when the two signals are of approximately equal magnitude.
Such prior tone detection circuits generally utilized resistance-inductance-capacitance components in the filters with individual tunning of the critical components. Even utilizing capacitors with temperature coefficients which would compensate for those of the inductors, it has been found difficult to maintain the accuracy of the center frequency to within the required tolerance of 0.3% over the short term and virtually impossible to maintain this accuracy over a long term period.